The present invention relates to a method and a device for determining the hardness of products such as fruit. More in particular, the invention relates to a method for determining the hardness of a product, for instance fruit, during a measuring cycle, comprising:
supporting the product,
tapping the product with a tapping device at at least one position of the product, as a result of which the product as a whole is set in its natural vibration and produces vibration signals,
sensing the vibration signals with a signal sensor, and
processing the vibration signals with a signal processing device, in which the hardness FRi of the product is determined, and also to a device for carrying out such a method.
It is known that the tapping of relatively hard products, for instance fruit, but also eggs, causes these products to resonate in one or several of its natural frequencies. The associated signals, in time domain or frequency domain, have been found characteristic of product groups such as apples of a specific variety, eggs, tomatoes, and so on. As soon as such products show damage or other deviations, significant changes occur in the signals.
Such a device is known from U.S. Pat. No. 5,696,325. Eggs lying on rollers of, for instance, a belt conveyor are tapped and thus set in vibration. From the acoustic signals sensed with a signal sensor, such as a microphone, it can be concluded whether the eggs show cracks or fractures. Such devices are used when selecting and sorting eggs.
A survey of such test methods for fruit is given in xe2x80x9cNew Developments in Fruit Quality Sortingxe2x80x9d, Itzhak Shmulevich, INTERPOMA 1998, Jun. 4-6, 1998. Tapping of products is referred to therein as xe2x80x9cforced vibrationsxe2x80x9d. U.S. Pat. No. 5,811,680 also shows an example thereof, in which the resonance signals caused by tapping of the product are sensed by piezoelectric signal sensors making contact with the product. In the above article the technique in which the hardness is determined via resonance vibrations is referred to, when compared with other techniques, as the most promising.
It has been found that in particular with respect to less hard products, by which the riper products are generally meant, these tapping techniques do not give unambiguous and reproducible results. Tapping on a small overripe spot on an otherwise sound product, or exactly tapping on a stalk, gives a similar drawback. The use of this technique makes selecting and sorting unreliable, or even impossible.
To remove this drawback, the present invention provides a method as indicated above, which is further characterized by
sensing the signals produced by the tapping device with a second signal sensor,
processing the vibration signals with a second signal processing device, the elastic properties of the product being determined in a volume range around the above at least one position.
determining the hardness FIi of the product,
comparing FIi and FRi and checking the comparison with a pre-adjusted value, the tapping and sensing with the second signal sensor being repeated when the comparison does not satisfy the pre-adjusted value.
In this field of the technique such a method is rather known. In the above article it is explained below xe2x80x9cdetection by impact forcexe2x80x9d how the course of the tapping or vibration itself, carried out on several spots of the product, also allows the determination of the hardness of such a product. Such a device is described in WO98/52037, in which the reaction force on the tapping element is measured. It is also indicated how such devices can be arranged on a conveyor belt, for instance on both sides of the belt, or also in a wheel rotating above the belt, which configurations allow several times of tapping of the products.
A drawback of such a configuration is not only that for several measurements correspondingly several devices to be mutually calibrated are used, but also that comparison with resonance measurements, and thus feedback, if required, is not possible.
With the device according to the invention the number of measurements can be advantageously limited, which is a necessity in view of the increasing transport and sorting velocities. Moreover, the measuring range is considerably extended with this combination. Sorting machines, or also desk-top arrangements equipped with the device according to the present invention, thus have the great advantage that they are suitable for many kinds of products, and where fruits are concerned, they have the advantage that with respect to a similar type both the hard and the less hard and riper specimen can be compared with each other.